1. Field of the Invention
The present invention relates to a fabrication method of three-dimensional (3D) microstructure, and more particularly, to a semiconductor process employing a thick photoresist by controlling exposure dosage to form a microstructure on a substrate.
2. Description of the Related Art
In recent years, microstructure devices have been developed for biochemistry, sensor science and pharmaceutics, and also have given rise to the advent of various fabrication techniques to fabricate microstructures made from various materials.
Embedded micro channels are generally used for microstructure devices to distribute and store micro fluid, and find applications in areas such as reagents, pharmaceuticals or inkjet printheads. There are at least four major methods that have been reported that use thick photoresist, such as SU-8 resist, to fabricate embedded micro channel structures. All the methods are superior to other non-mentioned micromachining techniques, such as excimer laser micromachining, especially in respect to costs and applicability.
As shown in FIGS. 1(a)–1(c), the first method discloses that a substrate 11 is overlaid with a filling material 13 and SU-8 layers 12 and 14 to define embedded micro channels. And then the filling material 13 is released from therein to obtain micro channels embedded in the substrate 11. UV light 15 is employed as an exposure source on SU-8 layers 12 and 14. However, this method must apply many different materials to finish one layer of embedded channel. In particular, the filling material 13 is different from the SU-8 layers 12, and negative effects in succeeding steps arise from their mismatching characteristics.
FIGS. 2(a)–2(c) illustrate the fabrication process of the second method. A whole SU-8 layer coated on the substrate 21 is directly overlaid with a metal mask 24 after exposed areas 22 and unexposed areas 23 arise from UV exposure. In the succeeding step, the metal mask 24 is coated with another SU-8 layer 25, and the SU-8 layer 25 is exposed to UV light 26. Therefore, the unexposed areas 23 of the lower SU-8 layer are not exposed to UV light 26 yet due to the metal mask 24. The micro channel can be released after the unexposed areas 23 are developed. Unfortunately, the metal mask 24 is a thin film liable to have cracks thereon. The cracks will cause a next stacked layer, such as SU-8 layer 25, failures. The root cause of the cracks is from the elevated temperature during an evaporation step or a succeeding baking step. Furthermore, the microstructure is also caused damage by the removal of the metal mask 24 thereafter.
The third method laminates a Riston film (dry film) 33 by a roller 34 on a SU-8 layer 32 formed on a substrate 31 to obtain micro channels, as show in FIGS. 3(a)–3(b). The adhesion uniformity problem between them is a serious concern during the laminating step. As shown in FIGS. 4(a)–4(c), a microstructure is formed on a substrate 41 by utilizing proton beam 44 to partially expose a SU-8 layer. The entire depth of SU-8 a layer can be fully exposed by a proton beam 44 with higher intensity. Therefore, the areas covered with a photo mask 46 are unexposed areas 43, the other areas are fully exposed areas 42. Under partial exposure of a proton beam 45 with lower intensity, the upper portions of the unexposed areas 43 become new exposed areas 42, and the lower portions of the unexposed areas 43 remain unexposed. In the fourth method, the proton beam 45 may be an elegant approach for dosage control on exposure, but not a popular source for common use.
In summary, the traditional methods for a microstructure either use more than two materials and a tedious process, or costly facilities like the proton beam, and are not simple enough for the fabrication of stacked channels.